1. Field of the Invention
The invention relates to a method which enables the operation of an aircraft turbo-engine to be continued, in an impaired manner, after the accidental occurrence of an unbalance in a rotor of the engine, such as may be caused by the breakage of a fan blade following impact by a foreign body. The invention also relates to a bearing support specially designed for use in carrying out the method.
An unbalance of an aircraft turbo-engine rotor causes a centrifugal rotational stress during operation which is transmitted to the structure of the turbo-engine through the bearings and bearing supports of the rotor, and through rotor-stator contact in the case of very considerable unbalances, and thence to the structure of the aircraft. There are two types of unbalances: inherent manufacturing unbalances and accidentally caused unbalances. Manufacturing unbalances, although not negligible, are reduced to relatively low values as a result of the care taken in the manufacture of rotors. Accidental unbalances originate chiefly from the breakage of blades and may be substantial, bringing about a rotational stress which may assume very high values requiring immediate shutdown of the turbo-engine in order to avoid the risk of destruction of the turbo-engine and aircraft structures. The breakage of a blade usually occurs close to the ground during the takeoff or landing of the aircraft, as a result of the accidental entry of foreign bodies such as birds into the air intake of the turbo-engine. Such an accident requiring the immediate shutdown of the engine may, in some circumstances, lead to the crashing of the aircraft. A first problem to be solved therefore is to keep the turbo-engine running in spite of an unbalance caused, for example, by the breaking of a blade, even if only for a limited period and with a reduced thrust, long enough for the aircraft to land.
Modern turbo-engines are generally of the bypass type and include a first stage of rotating blades termed "the fan" which provides the main part of the propulsion force, particularly in subsonic turbo-engines. These fan blades are very vulnerable to breakage because they are placed right at the front of the turbo-engine and because they are thin, of great size and are held by the rotor at one end only, the other end at the periphery of the rotor being free. Although a break will usually occur towards the free tip of the blade, the unbalance created may be substantial because of the great size of the blade. For example, in a large turbofan engine, the unbalance may reach 3 to 4 kg.m, which, taking into account the elasticity of the structure and the resonance phenomena resulting therefrom, can bring about a rotational stress of the order of 10.sup.5 DaN at 5000 rpm. A second problem therefore is to keep the turbo-engine running with such a large unbalance.
To improve the performance of a turbo-engine in normal operation, it is necessary to reduce air leaks between each stage of rotating blades and the inner wall of the casing facing the rotating blades. For this purpose, the clearance between the inner wall and the tips of the blades is increased, and this portion of the wall is lined with a soft material, referred to as abradable, to a thickness which is sufficient to interfere with the blades. When rotating, the blades exert a planing action on the abradable material and thereby adjust its thickness to the exact dimensions of the rotor, thus reducing air leakage to a minimum. A third problem is, therefore, to ensure that this leakage does not increase during the normal operation of the turbo-engine.
2. Summary of the Prior Art
A bladed rotor, and particularly a fan, inevitably possesses an inherent manufacturing unbalance which is all the more substantial if the fan is heavy and of great diameter. During rotation, this unbalance in conjunction with the natural elasticity of the turbo-engine structure brings about a radial and centrifugal rotational stress, with resonance points corresponding to the natural modes of the rotor and structure assembly, which would lead to an accelerated fatigue of the bearings and of the structure of the engine. To avoid this, the bearings of the rotor, and more particularly its front bearing, are usually held by a resilient bearing support which functions primarily to reduce vibrations and reduce the stresses transmitted to the engine structure. European Patent 63993 discloses such a bearing comprising a resilient element having a range of movement in a radial plane, the resilient element being in the shape of an open-work truncated cone forming a squirrel cage, and the movement of this resilient element being limited radially by a rigid element which is also in the shape of a truncated cone.
However, such a bearing suffers from the drawback of increasing the amplitude of the radial vibrations of the rotor, which causes an additional planing of the abradable material and hence air leakage. In practice, the designer must therefore limit the elasticity of such a bearing to the amount strictly necessary to reduce the vibrations resulting from the small manufacturing unbalance of the rotor. Consequently, this bearing does not provide a solution to the problems posed by accidentally occurring unbalance of the rotor.
U.S. Pat. No. 4,289,360 discloses a turbo-engine including a bearing support which is normally rigid but which can be released by the breakage of connection elements in response to the substantial unbalance resulting from a broken blade, the rotor rotating in a casing with an increased clearance filled in by a thicker layer of abradable material. The rotor then tends to rotate around its new axis of inertia, which reduces the unbalance and the stress exerted on the structure of the turbo-engine. However, such an arrangement can operate correctly only with efficient damping, and this is provided by segments which are movable relative to one another and are separated by oil films. This arrangement is not practical for the large turbo-engines used for aircraft, as they would require damping means of a mass and size which would be excessive, as well as substantial cooling means to protect the oil from overheating and eventual carbonisation.
Another known solution, which is particularly suited to the fan stage, again consists of equipping the turbo-engine with a bearing support designed to be released by the breakage of connection elements in response to a substantial unbalance resulting from a broken blade, the fan rotating in the casing with a smaller clearance filled by the abradable material. After the breakage of a blade and the release of the bearing, the fan, in tending to rotate around its new axis of inertia, planes away the abradable material so that the tips of the blades come to bear against the casing. The casing then guides the rotation of the fan and thus acts as a bearing. This solution allows the fan to be rotated for a very limited period of time, thereby obtaining a thrust from the engine, albeit a weak one. As the casing is rigid, this solution shifts the natural mode towards high rotational speeds, which lowers the rotational stress at low speeds, but increases the said rotational stress at high speeds. This stress may reach, for example, 10.sup.5 DaN at 5000 rpm. There are thus four drawbacks to this solution.
1) The maximum rotational stress occurs at the most critical moment in the flight of the aircraft, i.e. on take-off when the turbo-engines are operating at their maximum speed to supply maximum thrust.
2) Considerable energy is produced by the friction of the tips of the blades against the casing, and this energy may cause the blades to vibrate in a direction transversely to the blade, leading to the risk of further blades being broken and the danger of fire.
3) Dry friction of the tips of the blades against the casing is tolerable only for a very short period.
4) The casing must be reinforced to provide for its possible function as a bearing, resulting in an increase in the mass and cost.
Also known from U.S. Pat. No. 4527910 is a bearing support including a resilient squirrel cage element having its movement limited by a fixed ring, the energy thus produced being absorbed by a viscous damping. The fixed ring is itself capable of being freed by the breakage of its connection elements, thereby allowing a more substantial movement, and the energy of this is partly absorbed by a second viscous damping. As will be appreciated, such a solution is not practical for large aircraft turbo-engines, as the damping elements required would be prohibitive in terms of size and mass.